In response to Duane Wessels' question and Michael Shapiro's answer:
The GRASS graphics drivers use color lookup tables (reserving about 15
colors for vector "overlays"). This means that images with more than
240 colors for a 256 color display can't fit all its colors into the
hardware lookup table. In order to display the image a 6x6x6 color cube
is loaded into the hardware color registers (ie fixed mode) and the
nearest color is used instead of the "true" color. If you have an image
with a grey scale color map, and that image has more than 240 colors
(eg 1-255), then you get 6 levels of grey. The only solution is to
rescale the image data into the range 1-240 and set a grey scale for
the rescaled image.
I too use GRASS on a Sun (SunOS4.1.1,OpenWindows) but have found the fixed/
floating cut off point to be at 220. More importantly, if I try to rescale
an image using r.mapcalc to anything with more than 220 categories, I get
an error when I try to display the image.
eg. suppose I have a raster map with range 0-444, and I type the following
GRASS 4.0 > d.colormode float
GRASS 4.0 > r.mapcalc
mapcalc> newmap = oldmap/2
[range 0-222]
GRASS4.0 > d.rast newmap
GRASS will then display this as a floating colour image with the exception of
any cell values of 221 or 222, and give the message:
reset_color: can't set color 256
reset_color: can't set color 257
This error is repeated for the number of categories over 220.
Has anyone else experienced a similar problem, or know how to solve it ?
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Jo Wood
Project ASSIST email: JWO@UK.AC.LEICESTER
Midlands Regional Research Laboratory tel: 0533 523625
University of Leicester fax: 0533 522200
LEICESTER LE1 7RH
UK
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